San Andreas Fault: The Big One Is 'Inevitable'—but What Will Happen When It Hits?

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File Photo: San Francisco is pictured. Getty Images

If you live in California, you'll know the Big One is coming: a powerful earthquake of up to magnitude eight is headed for the state. Energy has been building up along the San Andreas Fault for more than a century. No-one knows exactly when or where, but that one day that energy will be unleashed.

It might strike at the heart of San Francisco, last devastated by a Big One in 1906. Or maybe it will tear through southern California like the magnitude 7.9 quake that hit in 1857 and ruptured some 225 miles of the San Andreas Fault.

More than 100 years on, it's hard to predict exactly how hard the next Big One will hit. John Vidale, director of the Southern California Earthquake Center and affiliate professor at the University of Washington, told Newsweek it won't look like in the movies—cities won't collapse into rubble and tsunamis probably won't sweep through California. But without adequate preparations, the Big One could "cripple" the finances of a state that just became the fifth largest economy in the world.

This interview has been edited for length and clarity.

What exactly is a "Big One," and where could such an earthquake hit?

A tectonic boundary between the North American and the Pacific plates cuts through California. It's a big fault where the two sides are moving three or four centimeters a year sideways. Strain builds up for one or two hundred years along that boundary, and then finally that strain becomes so great that the fault can't take it anymore. It breaks and moves 15 ft or so all at once, causing an earthquake.

There's three, four, five sections, to this fault—and many other faults running in parallel—but we worry about a Big One striking in the north or in the south of the San Andreas. There's a part between north and south in central California that seems act like a buffer. There's some chance a rupture could go end-to-end, but we think it's either unlikely or that it just doesn't happen.

How often do these massive earthquakes hit?

It's every few hundred years. The earthquakes that have happened in the meantime are still devastating to a local area, but instead of magnitude eight, they're more like magnitude seven. It's a logarithmic scale, so an eight has about 30 times more energy than a magnitude seven.

Don't smaller quakes help to dissipate some of the energy that's building up deep underground?

Those little earthquakes let out only a tiny amount of energy compared to the big ones. It would take 10 magnitude seven earthquakes to let out the strain of a magnitude eight. We don't have that many, so those little earthquakes hardly slow the big ones at all.

Does that mean the next big one is inevitable?

That's right. When we look at the history of the fault, we can see these big earthquakes have happened many times over the last few thousand years, so yeah, it's an inevitability. We just don't know if it's going to be now or two hundred years from now.

What kind of impact would a northern or southern California Big One have?

The impact of the northern big one would be tremendous—I mean the San Andreas runs right through San Francisco. It's quite a lot closer to San Francisco than it is to Los Angeles.

Downtown San Francisco is vulnerable—some of the oldest buildings survived the shaking back in 1906, but that doesn't mean they'd be safe in the next earthquake by any means. Many of the buildings are built close to the fault and on kind of soft ground that might liquify.

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File photo: Los Angeles pictured from above Getty Images

A southern Big One would likely strike a little further away from the heart of Los Angeles, so the impact might be smaller. On the other hand LA has a lot more stuff to break than San Francisco—a lot of it is pretty old. So I think the net expectation is similar north and south. The fault is further away in the South, but it's also riper, more ready, to go than the one in the north.

More generally, there's a lot of disasters that come from the strong shaking of an earthquake. It would certainly cause landslides, and conceivably chemical spills. We're also concerned about fires.

What about tsunamis?

Tsunamis aren't a big worry here. For an earthquake to make a tsunami it would have to be offshore—not be on the main part of the San Andreas. The ground would move sideways, not so much vertically as in other places, and it's hard to make a big wave moving sideways. But a lot of other things could happen.

When the Tōhoku earthquake hit Japan in 2011, it caused a disaster at the Fukushima Daiichi nuclear power plant. Are there are any nuclear reactors at risk from a Big One in California?

Not around here—there's one up by San Onofre but it's been turned off. There's been a lot of debate about nuclear reactors. Engineers argue they can make reactors safe, but there have been enough accidents over the years that for safety's sake they are tending not to build them in most places anymore.

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Workers move waste containing radiated soil, leaves and debris from the decontamination operation at a storage site in Naraha town, which is inside the formerly no-go zone of a 12 mile radius around the crippled Fukushima Daiichi nuclear power plant, August 24, 2013 Issei Kato/Reuters

So how prepared are cities like San Francisco and Los Angeles?

Both cities are among the best prepared for earthquakes in the U.S. There are codes for new buildings to make them stronger and more appropriate. We strengthen inspections especially at roads, bridges, ports, airports, hospitals and schools.

The issue is always that fixing problems costs a fortune—we can't just tear down all the buildings we know have problems and rebuild them. That would cripple the economy. So we're always making incremental steps to fix the worst problems at the moment that we can afford to address.

In California, when there's one of these big earthquakes, it won't be like in the movies—the cities won't turn to rubble. There is some damage and some deaths, but the larger issue for a city as a whole will be getting running again—the impact on the economy, the cost of replacing buildings and getting everything started again. I mean it's not as exciting—it doesn't make great Hollywood movies—but that's the biggest concern.

In other countries it's different. In China or Turkey or Iraq, the buildings are so bad that it really is terrifying when they're are all falling down around you. The San Andreas extends into Mexico. If the fault breaks there of course the country would feel a tremendous impact.

But in the U.S., most of the buildings will do okay. It's more the damage to the infrastructure and getting started again that's the problem.

If infrastructure doesn't rebound, what effects will this have on California?

It's certainly in the realm of possibility that the earthquake causes something that cripples the economy for a long time. Nobody expected the Fukushima reactor to be a dominant problem in Japan's 2011 earthquake, for example. There's always a small chance of some very serious unexpected problems.

It's also possible that a big earthquake might have less effect than we expect. It's just very hard to predict.

The Big One is worrisome for the government because it disrupts a large area. But for individuals, the moderate-size earthquakes that are right under our feet are often the worst threat.

Los Angeles, for example is filled with faults, and many of them could have a magnitude seven earthquake. A magnitude seven on a smaller fault might well do more damage than the Big One on the San Andreas. The Big One is only part of the danger here.

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File photo: Some of the mountains of California pictured from above. Getty Images

So, should people living in California be worried about a Big One?

It shouldn't be on people's mind every minute—there's enough things to worry about without dreading earthquakes—but every year people should make sure they're prepared. That means making sure they're not sleeping where things are going to fall on them. It means making sure their water heater is strapped to the wall so it doesn't fall out and break the gas line. It means a little bit of planning.

Fundamentally, you need to know what earthquakes are and how to react to them.

How can people protect themselves when they realize an earthquake has started?

The advice is to protect your head and chest—protect your personal safety. Duck, cover and hold is the standard advice and it's good advice. If possible get under a desk or table, if not then be aware of what's falling and make sure you're not in the path of things falling.

How are scientists working to warn people about earthquakes?

We figure we might be able to eliminate maybe 10 to 30 percent of the impact of an earthquake by warning people when the shaking is coming. We keep looking for clues to predict earthquakes—we see suggestions that we call "weak correlations" to indicate small changes in danger, but there's no giveaway about how big an earthquake will be, even when it's already started

With early warning systems, people can duck, cover and hold faster, factories can shift their machinery to make it safer, computer companies can adjust their programs so that there isn't so much disruption in financial markets, for example. There are a lot of little things we can do.

We think we can reduce the impact, we certainly can't eliminate it.